This invention relates generally to wideband cable television communications receivers having bandswitched input signal filters to reduce cross-modulation, intermodulation and other nonlinear distortions and particularly concerns techniques for reducing transmitted signal reflections and for preventing spurious signals emanating from such receivers from interfering with other receivers connected in the same cable network.
In a cable television (CATV) system, a large number of television channel signals are transmitted simultaneously from a headend to many subscribers via a cable network. The large number of television channel signals transmitted on such cable networks can result in degraded signal reception due to various modulation distortions such as cross-modulation, intermodulation and other nonlinear distortions. In addition, the increased number of interference signals emanating from each receiver such as reflections of the transmitted signal itself and spurious signals generated in the various receivers further degrades signal reception in the other receivers connected in the same cable network.
When processing two or more signals through nonlinear circuit devices such as typically found in communications receivers, various intermodulation distortions are produced which result in spurious beat products. In a CATV system, where a large number of evenly spaced channel signals are processed, these beat products tend to stack or accumulate. In particular, the third order intermodulation beat products thus accumulated are manifested as background noise in the reproduced video image. Since all the channel frequencies have some random deviation from the exact, evenly spaced channel frequencies, these distortion products are not exactly at the desired channel carrier frequency, but rather are clustered around the received channel carrier frequency, and furthermore they accumulate in terms of power as in the case of noise rather than in terms of voltage.
The other predominant intermodulation distortion is caused by second order intermodulation products which comprise either the sum or difference of two interfering channel signals. Second order intermodulation distortion manifests itself as a spurious beat in the reproduced video image which is more disturbing than the additional background noise as in the case of third order intermodulation distortions.
An equally severe limitation in extended bandwidth CATV reception results from interference signals emanating from each receiver. These interfering signal emanations include reflections of the transmitted signals giving rise to "ghost" images in other receivers connected in the cable network and other spurious signals emanating from the receivers of which the so-called "back talk" signals are the most serious. The latter are the product of the incoming signals the the receiver's first voltage controlled local oscillator (VCO) and fall within the reception frequency bandwidth causing intermodulation interference, or "beats", in the other receivers connected in the cable network.
For receivers used in earlier, narrower bandwidth CATV systems, the conventional circuits provided adequate dynamic range and the modulation distortions were held to a relatively low level. Also, due to the narrower bandwidth, the "back talk" fell outside of the CATV system bandwidth and caused no problem. There was thus no need to provide for selective filtering of incoming signals and the high input return loss over the entire frequency bandwidth did not present a problem in minimizing input refelections so as to prevent "ghost" problems in other receivers connected in the cable network.
For receivers designated for use in extended bandwidth CATV systems, an attempt has been made to lower the intermodulation distortions and the "back talk" interference by using a tunable lowpass filter. However, this technique still results in noticeable modulation distortions and "ghosts" in the signals transmitted in the cable network. Still another attempt to alleviate the foregoing problems is disclosed in copending application Ser. No. 544,947, entitled "Low Noise CATV Converter", filed in the name of the present inventor, which issued as U.S. Pat. No. 4,520,507 on May 28, 1985. In this system, a wideband RF amplifier and a PIN attenuator are provided to increase the dynamic range of the tuner used in the CATV receiver. This approach results in a substantially improved signal-to-noise ratio for low-level signals and a reduction in the modulation distortion of high level signals by limiting the maximum signal level applied to the first mixer stage of the receiver which is the primary source of such modulation distortion. However, a compromise is made in this approach due to the extemely tapered signal level condition encountered resulting in limitations in dealing with the composite second order intermodulations in such tapered signal level conditions.
The present invention is intended to overcome the aforementioned limitations of the prior art by providing a constant impedance bandswitched input filter for a signal receiver or converter coupled to an input cable in a wideband CATV system, wherein the front end of the receiver is impedance-matched over the entire bandwidth of the CATV system for reducing signal distortion and minimizing interference signals emanating from the various receivers in the cable network.